When two dissimilar metals are connected and that junction exposed to an elevated temperature, a thermal electromotive force is generated. This phenomenon is known as the Seebeck effect and is the basis of temperature measurements using thermocouples. When two similar metals are joined, no thermoelectric effect takes place. Theremorcouple leads are, therefore, made from the same material from which the thermocouples are made. Eventually, in the circuit, however, connections must be made to copper, such as at the binding post of a potentiometer or other measuring device. Unfortunately, these connections form two new junctions which are themselves thermocouples. The error produced by these new thermocouple junctions can be eliminated by placing the copper to thermocouple lead junction in a bath of melting ice. This keeps the two junctions at the same temperature and keeps that temperature constant. This cancels the thermoelectric effect of the new junction. If all additional terminations are copper to copper, no new thermal electromotive error is introduced. The electromotive force generated by the thermocouple is then measured with a potentiometer. The electromotive force generated by the thermocouple when an ice bath is used with the reference junction is represented by the equation: ##EQU1## where E equals electromotive force generated by the thermocouple; t equals temperature in degrees Celsius; A, B, C are constants dependent upon the materials used to fabricate the thermocouples. If the thermocouple to copper lead junctions are at some temperature other than the melting point of ice, then the above equation must be corrected by adding a term dependent upon the temperature of the junction. EQU E=E.sub.c +E.sub.r
where E equals electromotive force generated by thermocouple if an ice junction were used; E.sub.c equals correction factor; E.sub.r equals electromotive force actually measured. Once the electromotive force is determined, it is a simple matter to look up the temperature in a chart of electromotive force versus temperature which have been developed for various thermocouples and published in books such as the CRC Handbook of Chemistry and Physics.
Electronic devices are available to replace the ice bath for reference junction compensation. One such device is described by Ihlenfeldt et al. In U.S. Pat. No. Re. 30,735. The problem encountered when using a plurality of thermocouples is that a separate ice bath junction or electronic junction compensation device is needed for each thermocouple.
The present invention overcomes this problem by terminating all thermocouples at a common point to an isothermal block such that all terminations are at the same temperature. The temperature of the isothermal block which is used for termination is then measured. A method then can be used to simultaneously compensate all thermocouple signals as is disclosed in the Applicants' co-pending application, Ser. No. 398,491.